The invention relates generally to gas detectors, and, more particularly, to optical gas detectors.
Gas detectors are often used to detect a gas of interest under various conditions and in a variety of environments. For example, gas detectors may be used to monitor ambient air for the presence of flammable gases to protect firefighters or other emergency workers. Optical gas detectors are one type of gas detector that uses optical radiation to detect the presence and the concentration of a gas of interest. Specifically, optical radiation can cause gas molecules to oscillate. The oscillating molecules draw energy from the optical radiation and therefore absorb a portion of the optical radiation. The amount of radiation absorbed by the gas can be used to determine the presence and concentration of the gas of interest in the ambient air. Typically, an optical radiation source is used to emit optical radiation into a sample region containing a gaseous sample from the ambient air. A reduction of the intensity of the optical radiation due to the absorption of optical radiation by the gaseous sample is then detected at a detector positioned downstream from the sample region.
Early optical gas detectors typically include only a single radiation source and only a single detector. However, optical gas detectors that include only a single detector and only a single optical radiation source may only be able to provide reliable and unambiguous measurements under generally stable ambient conditions. Any change in the intensity of the optical radiation emitted by the optical radiation source, for example due to contaminants such as dust and/or a change in temperature and/or humidity, would appear to indicate the presence of the gas of interest. To overcome the measurement inaccuracies of early optical gas detectors, a second detector, referred to as a reference detector, may be used to carry out a parallel measurement of the radiation intensity at a different wavelength than the first detector, which is referred to as a measurement detector. The different wavelength is selected as a wavelength that will not be absorbed by the gas of interest, thereby providing a reference measurement that can be cross-calculated with the measurement of the measurement detector to provide a measure of the actual concentration of the gas of interest.
To further increase accuracy, a second, or reference radiation source may be used to provide four measurements instead of only two, sometimes referred to as “double-compensated” optical gas detectors. Specifically, the first, or measurement radiation source and the reference radiation source are arranged such that they emit radiation at different times or are modulated at two different frequencies. The optical radiation emitted from the measurement radiation source passes through the sample region, while the optical radiation emitted from the reference radiation source does not pass through the sample region. Accordingly, both detectors will receive an additional signal from the reference radiation source that is not affected by the gaseous sample. The additional signals provided by the reference radiation source serve as a measure of the relative sensitivity of each detector under the prevailing ambient conditions.
At least some known double-compensated optical gas detectors include a beam splitter positioned to direct radiation emitted from each of the radiation sources to both of the detectors. Specifically, the beam splitter is positioned relative to the reference radiation source such that a portion of the optical radiation emitted from the reference radiation source travels through the beam splitter to one of the detectors and another portion of the optical radiation emitted from the reference radiation source is reflected by the beam splitter to the other detector. In contrast, the detector that receives reference radiation reflected from the beam splitter receives radiation emitted from the measurement radiation source that has traveled through the beam splitter, while the detector that receives reference radiation that has traveled through the beam splitter receives measurement radiation that has been reflected from the beam splitter. However, changes in ambient conditions, for example due to contaminants such as dust and/or a change in temperature and/or humidity, may cause the beam splitter ratio to change. This may be a result of a change in the index of refraction and/or a change in the ratio of reflective area to transmissive area of the splitter. In a known arrangement, where the reference radiation and the measurement radiation are reflected from opposite sides of the splitter and where the transmitted portion of each radiation stimulates opposite detectors (of the reference and measurement pair of detectors), any change in beam splitter ratio affects the detectors inversely, which may be indistinguishable from a change in concentration of the gas of interest.
There is a need for an optical gas detector that is less sensitive to changing ambient conditions.